Optical fibers are key components in modern telecommunications systems and in recently developed remote sensing systems. Optical fibers are thin strands of glass capable of transmitting an optical signal over long distances with very low loss. In essence an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by one or more regions of cladding having lower indices of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the axis of the fiber with minimum attenuation. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
A typical optical fiber communications system comprises a source of optical input signals, a length of optical fiber coupled to the source and a receiver for optical signals coupled to the fiber. One or more amplifying devices are disposed along the fiber for amplifying the transmitted signal. Current systems use single wavelength digitally modulated optical signals. Contemplated systems would use a plurality of wavelengths in multichannel wavelength division multiplexed (WDM) systems. Optical filtering devices are useful components in optical fiber communications systems and especially in WDM systems.
There are a wide variety of sensing systems using optical fibers as sensing elements. See, for example, U.S. Pat. No. 4,761,073 issued to Meltz et al. on Aug. 2, 1988 which is incorporated herein by reference and U.S. Pat. No. 4,950,883 issued to W. Glenn on Aug. 21, 1990 which is also incorporated herein by reference. Such a system uses an optical fiber which includes along its length one or more photoinduced periodic gratings. In response to light transmitted along the fiber, each grating acts as a partial reflecting mirror for light of a narrow band of wavelengths corresponding to twice the periodicity of the grating. Stress or temperature change applied to the fiber in the grating region changes the effective periodicity of the grating and thereby shifts the wavelength of the light which is reflected by the grating. Thus spectral analysis of the reflected light provides information concerning conditions at the location of the gratings. Accordingly, wavelength shift detectors are useful components in optical fiber sensing systems.